In wireless communication networks, it is known to use multiple-input multiple-output (MIMO) technologies for providing enhanced capacity and/or performance. For example, usage of a reciprocity based time division duplex (TDD) massive MIMO technology is being considered in the case of 5G (5th generation) mobile communication network.
Massive MIMO is a multi-user MIMO technology where each base station (BS) is equipped with a large number of antenna elements, typically 50 or more, which are used to serve many terminals that share the same time and frequency band and are separated in the spatial domain. One assumption in the massive MIMO technology is that there are more BS antennas than terminals, at least twice as many, but preferably even more. The massive MIMO technology offers many benefits over conventional multi-user MIMO. In the massive MIMO technology, the large excess of antennas over active terminals allows for enhanced beamforming performance and good scalability in view of served terminals.
By operating in TDD mode, the massive MIMO technology may utilize channel reciprocity, i.e., assume that radio channel characteristics are the same in both an uplink transmission direction from the terminal to the BS and a downlink transmission direction from the BS to the terminal. Channel reciprocity allows the BSs to acquire channel state information (CSI) from reference signals transmitted by the terminals in the uplink direction, e.g., from Sounding Reference Signals (SRS). This CSI may then be utilized for controlling beamforming both in the uplink direction and the downlink direction.
However, using reciprocity based beamforming in a massive MIMO technology typically requires high accuracy of the CSI acquired by the BS. On the other hand, performing channel estimation individually on each antenna element results in a very low signal to noise ratio (SNR) or signal to interference plus noise ratio (SINR), because there is no MIMO gain. It is thus a very demanding task to filter the contribution of the reference signal from a background of noise and interference.
Accordingly, there is a need for techniques which allow for efficient radio channel estimation based on a reference signal received by a large number of antennas.